In oil well evaluation, quantitative analyses of formation fluid are typically performed in a laboratory environment, the samples having been collected downhole and brought to the surface in the sample chamber of a wireline formation tester.
Standard laboratory procedures are available to do quantitative analyses by addition of a reagent that reacts chemically with a specific target species in a sample to cause detectible changes in fluid property such as color, absorption spectra, turbidity etc. See Vogel, A. I., “Text-Book of Quantitative Inorganic Analysis, 3rd Edition”, Chapter 10-12, John Wiley, 1961. Such changes in fluid property may be caused, for example, by the formation of a product that absorbs light at a certain wavelength, or by the formation of an insoluble product that causes turbidity, or bubbles out as gas. For example, addition of pH sensitive dyes is used for calorimetric pH determination of water samples. A standard procedure for barium determination requires addition of sodium sulfate reagent to the fluid sample resulting in a sulfate precipitate that can be detected through turbidity measurements. Some of these standard laboratory procedures have been adapted for flow injection analysis (Ruzicka, J. and Hansen, E. H., Flow Injection Analysis, Chapters 1 and 2, John Wiley, 1981). Flow injection analysis “is based on the injection of a liquid sample into a moving non-segmented continuous carrier stream of a suitable liquid” (Chapter 2, page 6).
Fluid samples collected downhole can undergo various reversible and irreversible phase transitions between the point of collection and the point of analysis as pressure and temperature conditions are hard to preserve. Concentrations of constitutive species may change because of loss due to vaporization, precipitation etc., and hence the analysis as done in the laboratories may not be representative of true conditions downhole. For example, water chemistry and pH are important for estimating scaling tendencies and corrosion; however, the pH can change substantially as the fluid flows to the surface. Scaling out of salts and loss of carbon dioxide and hydrogen sulfide can give misleading pH values when laboratory measurements are made on downhole-collected samples.
While downhole formation sampling tools are usually equipped with spectrophotometric detectors, currently there are no available methods to carry one or more reagents downhole and inject them into the flow-line to enable such an analysis.